Composite fibers and microfibers made therefrom as well as different processes for their manufacture are well known in the art.
The composite fibers are manufactured in general by combining at least two incompatible fiber-forming polymers via extrusion followed by optionally dissolving one of the polymers from the resultant fiber to form microfibers.
U.S. Pat. No. 3,700,545 discloses a multi-segmented polyester or polyamide fiber having at least 10 fine segments with cross sectional shapes and areas irregular and uneven to each other.
The spun fibers are treated with an alkali or an acid to decompose and at least a part of the polyester or polyamide is removed.
Described is a complex spinnerette for the manufacture of such fibers.
U.S. Pat. No. 3,382,305 discloses a process for the formation of microfibers having an average diameter of 0.01 to 3 microns by blending two incompatible polymers and extruding the resultant mixture into filaments and further dissolving one of the polymers from the filament. The disadvantage of this process is, that the cross section of these filaments is very irregular and uneven, so that the resulting microfibers are irregular, uneven and having varying diameters.
U.S. Pat. No. 5,120,598 describes ultra-fine polymeric fibers for cleaning up oil spills. The fibers were produced by mixing an polyolefin with poly (vinyl alcohol) and extruding the mixture through a die followed by further orientation. The poly (vinyl alcohol) is extracted with water to yield ultra-fine polymeric fibers. The disadvantage of this process is that the cross section is irregular and uneven which is caused by the melt extrusion and what results in irregular and uneven microfibers and the islands, which form the microfibers after the hydrolysis, are discontinuous, which means that they are not continuous over the length of the composite fibers.
EP-A-0,498,672 discloses microfiber generating fibers of island-in-the-sea type obtained by melt extrusion of a mixture of two polymers, whereby the sea polymer is soluble in a solvent and releases the insoluble island fiber of a fineness of 0.01 denier or less. Described is polyvinyl alcohol as the sea polymer. The disadvantage is that by the process of melt mixing the islands-in-the-sea cross section is irregular and uneven and the islands, which form the microfibers after the hydrolysis, are discontinuous, which means that they are not continuous over the length of the composite fibers.
Object of the present invention is to provide a composite fiber with a cross-section having at least 19 segments of a polyolefin which is water-insoluble, surrounded by a water-soluble polymer, wherein the segments of the polyolefin are uniformly distributed across the cross-section of the composite fiber and are continuous over the length of the composite fiber.
Another object was to provide a process for the manufacture of such a composite polyolefin fiber.
Another object was to provide a process for the manufacture of polyolefin microfibers of a fineness of not greater than 0.3 denier from the composite fibers.